The Organic Light Emitting Display (OLED) possesses many outstanding properties of self-illumination, low driving voltage, high luminescence efficiency, short response time, high clarity and contrast, near 180° view angle, wide range of working temperature, applicability of flexible display and large scale full color display. The OLED is considered as the most potential display device.
The OLED display device comprises a plurality of pixels aligned in array. The pixel drive circuit is utilized to drive the organic light emitting diode to emit light. FIG. 1 shows a 3T1C pixel drive circuit of an OLED, comprising: a first thin film transistor T1, a second thin film transistor T2, a third thin film transistor T3, a storage capacitor Cst and an organic light emitting diode D. In comparison with traditional 2T1C pixel drive circuit, the third thin film transistor T3 is added in the 3T1C pixel drive circuit. The second thin film transistor T2 is a drive thin film transistor, and a gate and a source of the second thin film transistor T2 are respectively coupled to a first node A and a second node S; the first thin film transistor T1 is employed to charge the first node A, i.e. the gate of the second thin film transistor T2; a gate of the third thin film transistor T3 receives a discharge control signal DCS, and a source is electrically coupled to the first node A, and a drain receives a reference voltage line Vref, and the third thin film transistor T3 is employed to discharge the first node A, i.e. the gate of the second thin film transistor T2. After the gate of the third thin film transistor T3 is on to charge the node A, and the voltage is stable, the node A voltage is about the reference voltage Vref, which acts to discharge the organic light emitting diode D. That is to say, the voltage value of the node A after discharge can be controlled by controlling the value of the reference voltage Vref.
The formula of calculating the current I flowing through the organic light emitting diode is:I=k(VGS−Vth)2=k(VA−VS−Vth)2 wherein k is an intrinsic conductive factor of the drive thin film transistor, i.e. the second thin film transistor T2, and VGS is a gate-source voltage of the second thin film transistor T2, and Vth is a threshold voltage of the second thin film transistor T2, and VA is the voltage of the first node A, i.e. a gate voltage of the second thin film transistor T2, and VS is a voltage of the second node S, i.e. a source voltage of the second thin film transistor T2.
The Pulse-Width Modulation (PWM) drive method is to control the duration of charge and discharge of respective sub frames in one frame by controlling the on-periods of the first thin film transistor T1 and the third thin film transistor T3. With combination that the sense of the human eyes to the brightness is the integral principle of time, the digital voltage (i.e. two Gamma voltages) can be utilized for showing pictures of various gray scale brightnesses.
In prior art, the reference voltages Vref of all the OLED pixel drive circuit are generally set to be some constant value. Namely, the reference voltages Vref of the pixel drive circuits of respective rows are equal. Under such circumstance, as shown in FIG. 2, the display brightness (Luminance) of the OLED increases from the first luminance L1 to the second luminance L2 along with the increase of the Gray level of the display frame. The contrast of the display frame can be calculated and obtained from the formula Contrast=L2/L1. Ultimately, the obtained frame contrast is lower.